A big write shouldn't take down the node
Reads on SQE spill to disk when they run out of memory. Writes did not. A large CTAS, a wide MERGE, or an oversized client upload could balloon a coordinator buffer past its memory limit and get the process OOM-killed, which takes every other query on that node with it. We gave the write path the same memory discipline the read path already had: pool-track the buffers that must exist so an oversized write fails as one typed error, and stream the ones that never needed to buffer at all.
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